WASHINGTON — Signals that humans are the main factor behind recent global warming are stronger than ever, an authoritative global scientific report will warn when it is released next week.

A draft of the United Nations report by 600 scientists says it is "very likely" that human activities are the main cause of warming in the past 50 years, strengthening a conclusion in their last study in 2001 that the human link was "likely".

"It is very likely that ... greenhouse gas increases (from human activities) caused most of the globally average temperature increases since the mid-20th century," one source who had seen the draft quoted it as saying.

The 2001 report defined "very likely" as a 90-99 percent probability and "likely" as a 66-90 percent chance.

The new report means narrower ground for skeptics to argue that natural variations, such as in the sun's output, are to blame rather than emissions from burning oil, coal and gas.

Extremes narrowedThe draft does have some good news in that improved computer modeling allowed scientists to narrow the scenarios for projected future temperature and sea level rise.

The draft projects that world temperatures are likely to rise by 3.6-8.1 Fahrenheit by 2100 unless the world manages drastic cuts in greenhouse gas emissions from factories, cars and power plants.

The 2001 report had projected a wider possible range of between 2.5-10.4 F.

Sources said the new report would also narrow both ends of the band of projected sea level rises, projected in the 2001 report at between 3.5-35 inches by 2100. Details of the new figures were not available.

"There's good news that the top extremes for temperature and sea level rises have been cut," one source said. "But anyone hoping that the rise will be at the bottom end of the range will be disappointed."

Many scientists also stress that sea level rise beyond 2100 could be catastrophic if the ice sheets in Antarctica and Greenland start to lose significant mass.

'Smoking missiles'The draft being released next week is actually the first of four chapters by the U.N. Intergovernmental Panel on Climate Change.

Several scientists on Monday spoke on the record about the upcoming first chapter, refusing to release details but speaking in broad terms about what it would reflect.

“We have barely started down this path” of warming, said chapter co-author Richard Alley of Penn State University.

“The smoking gun is definitely lying on the table as we speak,” added U.S. climate scientist Jerry Mahlman, who reviewed all 1,600 pages of the first segment of the giant four-part report. “The evidence ... is compelling.”

Andrew Weaver, a Canadian climate scientist and chapter co-author, went even further: “This isn’t a smoking gun; climate is a battalion of intergalactic smoking missiles.”

The first chapter, written by more than 600 scientists and reviewed by another 600 experts and edited by bureaucrats from 154 countries, includes “a significantly expanded discussion of observation on the climate,” said co-chair Susan Solomon, a senior scientist for the U.S. National Oceanic and Atmospheric Administration.

That report will feature an “explosion of new data” on observations of current global warming, Solomon said.

The 12-page summary for policymakers will be edited in secret word-by-word by government officials for several days in Paris next week and released to the public on Feb. 2.

The other chapters will be issued in phases over the year, as was the case with the last IPCC report, issued in 2001.

Global warming is “happening now, it’s very obvious,” said Mahlman, a former director of NOAA’s Geophysical Fluid Dynamics Lab who lives in Boulder, Colo. “When you look at the temperature of the Earth, it’s pretty much a no-brainer.”

Expect the ‘iconic statement’
Look for an “iconic statement” — a simple but strong and unequivocal summary — on how global warming is now occurring, said one of the authors, Kevin Trenberth, director of climate analysis at the National Center for Atmospheric Research, also in Boulder.

The February report will have “much stronger evidence now of human actions on the change in climate that’s taken place,” Rajendra Pachauri told the AP in November. Pachauri, an Indian climatologist, is the head of the international climate change panel.

An early version of the ever-changing draft report said “observations of coherent warming in the global atmosphere, in the ocean, and in snow and ice now provide stronger joint evidence of warming.”

And the early draft adds: “An increasing body of evidence suggests a discernible human influence on other aspects of climate including sea ice, heat waves and other extremes, circulation, storm tracks and precipitation.”

Raising the temperature
The world’s global average temperature has risen about 1.2 degrees Fahrenheit from 1901 to 2005. The two warmest years on record for the world were 2005 and 1998. Last year was the hottest year on record for the United States.The report will draw on already published peer-review science.

Some recent scientific studies show that temperatures are the hottest in thousands of years, especially during the last 30 years; ice sheets in Greenland in the past couple years have shown a dramatic melting; and sea levels are rising and doing so at a faster rate in the past decade.

Also, the second part of the international climate panel’s report — to be released in April — will for the first time feature a blockbuster chapter on how global warming is already changing health, species, engineering and food production, said NASA scientist Cynthia Rosenzweig, author of that chapter.

More computer models
As confident as scientists are about the global warming effects that they’ve already documented, they are as gloomy about the future and even hotter weather and higher sea level rises. Predictions for the future of global warming in the report are based on 19 computer models, about twice as many as in the past, Solomon said.

In 2001, the panel said the world’s average temperature would increase somewhere between 2.5 and 10.4 degrees Fahrenheit and the sea level would rise between 4 and 35 inches by the year 2100.

The 2007 report will likely have a smaller range of numbers for both predictions largely because of improvements in the climate models, Pachauri and other scientists said.

"I think probably the low value (in the 2001 report) and also the high value came from models that probably had mistakes in them," Trenberth recently told the Rocky Mountain News of Denver. "The confidence in those numbers was probably not that good, and they probably never should have been used in the way in which they were used."